As Plasmodium falciparum malaria evolves and emerges in new geographic regions, there is an urgency to understand factors that lead to clinical pathology and the development of immunity. P. falciparum emerged in the Amazon Jungle city of Peru, Iquitos, in 1994, and since 1998 it has been sustained in low transmission. In 2003, we entered into a strong partnership of the Universidad Nacional de la Amazonia Peruana (UNAP), Iquitos, Peru. We have an existing R01, R01 AI064831 (years 2005-2010), to study epidemiologic, immunologic and genetic factors that might influence the development of clinical immunity to P. falciparum malaria in a low transmission setting. A critical non-explored extension of the R01 is to test how erythrocyte invasion might contribute to the P. falciparum pathogenesis and immunity observed in this cohort. Culture adapted strains of P. falciparum can use multiple redundant and interchangeable invasion pathways to enter human erythrocytes. It has been hypothesized that this diversity of available pathways is a key feature in P. falciparum pathogenesis, allowing parasites to adapt rapidly to different erythrocyte genotypes and perhaps allowing some isolates to multiply more rapidly than others in vivo. The unique malaria transmission situation in Iquitos and the facilities and researchers at UNAP give us a breakthrough opportunity to study P. falciparum erythrocyte invasion and its impact on pathogenesis. While advancing in molecular parasitology, we will test our central hypothesis: isolates that depend on a limited number of invasion pathways are associated with asymptomatic infections that decrease in parasite density without treatment. With the abilities, investment and project-interest of UNAP, we have already culture- adapted 58 Peruvian isolates. However, this research must be undertaken on fresh blood samples that have not undergone selection in culture. Also, the characteristics of each isolate must be linked to the clinical, genetic and immunologic data of each infection, provided by the R01 study. From P. falciparum isolates from fresh blood collections of malaria patients, we will (1) investigate the extent of variation in erythrocyte invasion pathways, (2) establish the invasion ligand expression profile and genotype of the phenotyped isolates, and (3) correlated clinical symptoms and specific erythrocyte invasion pathways, ligand expression profiles or ligand genotypes. This research will be carried out primarily in Peru at UNAP, in collaboration with Dr. Jean Hernandez, the Peruvian lead researcher on this proposal. In addition to complementing our existing R01, the aims of this proposal are a vehicle for valuable technology transfer, are a high and independently identified priority for UNAP, are facilitating training and advancement of other researchers at UNAP, and are certain to generate important resources and scientific knowledge. Public Health Relevance: Plasmodium falciparum parasites cause up to 3 million deaths from malaria each year, and malaria is expanding to new geographic regions, entering the Amazon Jungle region of Peru in 1994. This unique malaria transmission situation and our existing collaboration with the Universidad Nacional de Amazonia Peruana (UNAP), Iquitos, enables us to test how P. falciparum erythrocyte invasion occurs and might contribute to malaria pathogenesis and immunity. By understanding the pathways P. falciparum can use to enter erythrocytes, we can develop intervention strategies that block these pathways, decrease human disease and enhance the development of human immunity.